Globally, young women represent one of the most vulnerable groups at risk for HIV acquisition highlighting the need for safe, acceptable and effective prevention products. Outcomes in pre-exposure prophylaxis (PrEP) clinical trials have been uniformly disappointing in this high-risk age group, reflecting both behavioral and biological characteristics. The efficacy of PrEP reflects a balance between host susceptibility to HIV and the concentration of drug present in host target cells at the time of exposure. One of the most important biological factors that modulates both the risk of HIV acquisition and the pharmacokinetics and efficacy of topically delivered PrEP products is the vaginal microbiome. Recent studies highlight the complex mechanisms by which individual bacteria and their metabolic products adversely affect the pharmacokinetics of several different PrEP drugs by competing with human cells for drug uptake, inhibiting drug transport into human cells, or metabolizing drugs. Moreover, bacterial vaginosis, which is common in adolescent and young women, is associated with increased HIV risk, possibly reflecting mucosal inflammation. However, the precise mechanisms linking dysbiosis with inflammation and the cumulative effect of the microbiome on PrEP pharmacokinetics are not defined. This application will address this critical knowledge gap and test the overarching hypothesis that vaginal dysbiosis in adolescent and young adult women reduces PrEP efficacy by promoting mucosal inflammation, increasing HIV risk, and decreasing local drug bioavailability. We will use cutting edge technologies including flow cytometry based bacterial cell sorting of immunoglobulin-coated bacteria combined with 16S rRNA sequencing (IgSeq), metagenomic and metatranscriptomic sequencing to evaluate the link between vaginal dysbiosis and mucosal inflammation. Vaginal swabs will be collected from adolescent and young women with symptomatic bacterial vaginosis (BV) before and after standard of care treatment and from asymptomatic controls (no BV) who are frequency matched for age, race/ethnicity and contraceptive use. We will characterize the total bacterial population and the IgA and/or IgG coated and uncoated bacteria and correlate findings with measures of genital tract inflammation including gene expression in vaginal biopsy tissue. We predict that the composition of Ig coated bacteria will differ before and after BV treatment and compared to controls and will identify bacteria that drive genital tract inflammation. Using the clinical samples, we will determine the cumulative effects of bacterial communities and their metabolome on tenofovir-based PrEP (including tenofovir and its prodrugs), assess which mechanisms dominate, and explore potential interventions that might promote more consistent drug pharmacology. We will also evaluate ?next-generation? PrEP products including integrase inhibitors. Together, these results will provide rationale for the selection, dosing and formulation of drugs alone or in combination for optimal prevention of HIV in young women.